Field of the Invention
The present invention relates to an ultrasonic imaging apparatus and an ultrasonic imaging method for performing diagnosis of organs in a living body or nondestructive inspection by transmitting and receiving ultrasonic waves.
Description of a Related Art
Generally, in an ultrasonic imaging apparatus, an ultrasonic probe including plural ultrasonic transducers having functions of transmitting and receiving ultrasonic waves is used. When an ultrasonic beam formed by synthesizing plural ultrasonic waves is transmitted from the ultrasonic probe toward an object to be inspected, the ultrasonic beam is reflected in a region having different acoustic impedances, that is, a boundary between tissues within the object. Thus generated ultrasonic echoes are received and images are formed based on the intensity of the ultrasonic echoes, and thereby, the status within the object can be reproduced on a display screen.
In a conventional ultrasonic imaging apparatus, there is used a method of obtaining information on the entire region of interest by estimating reflection signal information on an ultrasonic beam based on ultrasonic echoes obtained by the reflection of the ultrasonic beam formed within the object, and sequentially changing the direction of the ultrasonic beam. In order to display this in a typical monitor, so-called scan conversion is performed.
FIG. 20 is a diagram for explanation of transmission beam forming. In a method called electronic scan, a group of arranged vibrators are used. When high-frequency voltages (drive signals) are applied to the respective vibrators included in the group of vibrators, ultrasonic waves are introduced into the object. At this time, as shown in FIG. 20, since the times when the respective vibrators are driven are varied, lags are caused in ultrasonic waves to be introduced into the object. When the respective vibrators are sufficiently small, the ultrasonic waves generated by the respective vibrators spherically propagate within the living body, and regions where phases of ultrasonic waves match one another and the ultrasonic waves are mutually intensified are formed because of the lags.
As shown in FIG. 20, when a focal point is formed in a certain position, the transmission times may be adjusted such that the wavefronts from the respective vibrators are located on concentric circles around the point. In the focal position, the phases of the ultrasonic waves transmitted from all vibrators match one another, and the ultrasonic waves are mutually most intensified. Similarly, in another position, ultrasonic waves from part of vibrators are intensified. Alternatively, in other positions, regions where ultrasonic waves are mutually weakened due to phase reversal are formed. As a result, so-called side lobes are formed. As described above, not only an ultrasonic acoustic field having a beam-like form exists, but also some acoustic fields are formed in the all regions.
FIG. 21 is a diagram for explanation of reception beam forming. The mechanism of reception beam forming is the same as that of the transmission beam forming. At reception of ultrasonic echoes, it is assumed that ultrasonic echoes are reflected by a reflector and spherically spread from the reflector. Accordingly, also at the reception, the reception signals from the reflector are mutually most intensified by providing the same delays as those at transmission to the reception signals outputted from the plural vibrators and adding those reception signals to one another.
Here, at the same times when ultrasonic echoes from the reflector reach the respective vibrators, if ultrasonic echoes from another sound source reach the vibrators, the ultrasonic echoes from the other sound source are also mutually intensified. That is, what are included in the added reception signals are not necessarily the ultrasonic echoes only from the reflector of interest, and the ultrasonic echoes from the other sound source become noise. When the transmission ultrasonic beams exist only on the straight line connecting the vibrator and the reflector, this noise does not exist. However, in the conventional transmission system as described above, it is impossible to form such a transmission ultrasonic beam, and mixture of noise due to acoustic fields is not avoidable.
In FIGS. 20 and 21, the ultrasonic beams in a direction orthogonal to the vibrator surfaces are shown, however, when the vibrators are sufficiently small as described above, the transmitted ultrasonic waves spread as a point sound source in all directions, and further, ultrasonic echoes from all directions can be received at reception. As shown in FIG. 22, when the amounts of delay are increased to be asymmetric at the right and left sides (vertically in the drawing), an ultrasonic beam can be tilted. FIG. 22 shows reception, but the same is true at transmission. Generally, in a method called sector scan, this transmission and reception method is used with extremely small vibrator widths.
Although the acoustic fields are formed in the all regions as described above, the ultrasonic beam transmitted and received is narrow, the reception signals mainly obtained represent information from a limited region on the ultrasonic beam. Accordingly, in order to obtain tomographic images, the ultrasonic beam should be sequentially focused on the entire region of interest, and a method of shifting the ultrasonic beam is adopted.
FIG. 23 shows shifting of an ultrasonic beam in the case of linear scan. As shown in FIG. 23, the ultrasonic beam is shifted by changing the groups of vibrators that contribute to transmission and reception. However, according to the method, spacings of the ultrasonic beams are equal to the spacings of the vibrators and the resolving power of tomographic images depends on the beam spacings, and therefore, the obtained image resolving power is not sufficient. Accordingly, various methods are devised in order to obtain more ultrasonic beams. FIG. 24 shows an example, called a micro-angle method. According to the micro-angle method, the symmetry in amounts of delay is eliminated even using the same group of vibrators, and thereby, beams slightly different in directions can be obtained. Further, in the case of sector scan, a method of changing the direction of the beam by changing the amounts of delay in FIG. 22 is adopted.
The information on the ultrasonic beam obtained in this manner is directly depicted on an X-Y monitor in an early ultrasonic imaging apparatus. However, in view of data recording or the like, the information is currently depicted on a common monitor by using a method of scan conversion.
FIG. 25 shows an example of scan conversion in the case of linear scan. In the scan conversion in the case of linear scan, the processing of converting data on points indicated by diamond marks in ultrasonic beam coordinates into data on point (X,Y) indicated by a circle in X-Y coordinates on a monitor is performed. FIG. 26 shows an example of scan conversion in the case of sector scan. In the scan conversion in the case of sector scan, the processing of converting data on points indicated by diamond marks in ultrasonic beam coordinates into data on point (r, φ) indicated by a circle in polar coordinates, that is, on point (X,Y) indicated by a circle in X-Y coordinates on a monitor is performed.
When a micro region is seen, if the number of data of ultrasonic beams is larger than the number of data in X-Y coordinates, thinning of the data is necessary. On the other hand, if the number of data of ultrasonic beams is smaller than the number of data in X-Y coordinates, addition for the lack of data is necessary. Here, improper thinning causes aliasing. Further, in addition of data, the lack of data is created by interpolation processing from the data of surrounding ultrasonic beam has been mainly adopted, but the interpolation processing is one kind of image processing and just the estimation of unknown information based on surrounding information.
As described above, in the conventional ultrasonic imaging apparatus, it has been necessary to repeat transmission and reception of ultrasonic waves in various beam directions at many times to obtain tomographic images. Further, the scan conversion processing does not necessarily reproduce precisely the reflective sound source information of the living body, but means for obtaining good image quality by a kind of artificial processing. Furthermore, the configuration of the ultrasonic beam depends on the geometric relationship, and thereby, there is a disadvantage that the images to be obtained may differ depending on the setting of the sound speed within the living body.
As a related conventional technology, in U.S. Pat. No. 6,685,645 discloses systems and methods of obtaining an image using a broad beam at one ultrasonic transmission. However, in U.S. Pat. No. 6,685,645, a specific method of estimating tomographic images is necessarily not disclosed.
Further, Japanese Patent Application Publication JP-A-11-89846 discloses an ultrasonic imaging apparatus intended for higher image quality by improving the S/N ratio. In the ultrasonic imaging apparatus, reception signals are converted into complex signals by orthogonal detector, filtering is performed on the real parts and imaginary parts, amplitudes are calculated based on the real parts and the imaginary parts, and images are displayed based on the amplitudes. However, in order to obtain the reception signals, plural times of ultrasonic transmission and reception are executed with respect to one direction.